The present invention relates to novel iron sulfides having excellent durability, processes for producing the same, iron sulfide mixture, a heavy metal treating agent containing either of these novel iron sulfides as an effective component, and a method by which wastes containing various heavy metals are made harmless with the heavy metal treating agent.
Reviews of Pure and Applied Chemistry, Vol.20, pp. 175-206 (1970) shows structures of iron sulfides. It discloses that the generally known iron sulfides for industrial use and the iron sulfide produced by melting a mixture of an iron powder and sulfur have a pyrrhotite structure, while the iron sulfide yielded by mixing a solution containing iron(II) ions with a solution containing sulfur ions has a mackinawite structure.
A technique in which wastes containing various heavy metals are made harmless with an iron sulfide is widely known. For example, a method for treating harmful heavy metals, e.g., Pb, Cd, Cr, Hg and As, in an aqueous solution with an iron sulfide for industrial use (pyrrhotite structure) is disclosed in, e.g., Japanese Patent Publication No. 43472/1974and Japanese Patent Laid-Open Nos. 31806/1972, 13294/1975, 96053/1975, 126685/1977 and 227881/1985.
A technique is also known in which an iron sulfide (mackinawite structure) prepared by mixing a solution containing iron(II) ions with a solution containing sulfur ions is used to treat heavy metals. The iron sulfide obtained by this process is known to have the higher ability to treat heavy metals than iron sulfides for industrial use. For example, Japanese Patent Laid-Open Nos. 11291/1973, 31152/1974, 113559/1977, 148473/1977 and 102273/1978 disclose a method of treating harmful heavy metals with a solution containing iron(II) ions and a solution containing sulfur ions or with an iron sulfide obtained by mixing these solutions.
However, the iron sulfide having a mackinawite structure is so susceptible to oxidation that it reacts with moisture and oxygen in the air and thereby decomposes into sulfur and iron(III) hydroxide. Namely, the ability of this iron sulfide to treat heavy metals readily decreases. Because of this, treatment with the iron sulfide having a mackinawite structure has hitherto been conducted generally in such a manner that a solution containing iron(II) ions is mixed with a solution containing sulfur ions to prepare a slurry containing the iron sulfide and this slurry is immediately mixed with a waste to be treated, such as a wastewater. Although the solution containing sulfur ions which is usually employed is an aqueous solution of sodium sulfide or sodium hydrosulfide from the standpoints of cost and industrial availability, it is necessary to handle the solution by a skilled person having a knowledge of chemistry because of the harmfulness, corrosiveness, and offensive odor of the solution, etc. Namely, the solution is generally difficult to handle. Furthermore, there has been the following problem. In the case where the iron sulfide is used as a slurry, heightening the concentration of the iron sulfide is difficult because of the necessity of imparting a certain level of flowability. This limitation on iron sulfide concentration results, for example, in an increased transportation cost when the iron sulfide slurry is produced in, e.g., a factory. Moreover, when the iron sulfide is added in a large amount to treat a fly ash or soil containing heavy metals in a high concentration, the waste thus treated has too high a water content and is hence difficult to handle thereafter, although this problem is not aroused in the treatment of wastewaters and the like.
On the other hand, in the case where a powder of mackinawite iron sulfide is prepared from the slurry through filtration and drying, there have been the following problems. It is necessary to conduct the operation in an inert atmosphere or to add an antioxidant in order to prevent oxidation. The iron sulfide powder obtained should be stored in a container impermeable to oxygen and moisture in order to prevent the powder from oxidatively deteriorating. Alternatively, it is necessary to add a reducing agent to the powder so as to prevent oxidation. Even though a reducing agent is added, this does not basically eliminate the susceptibility to oxidative deterioration and the powder still has considerably poor storage stability because the iron sulfide begins to oxidize when the reducing agent has been consumed.
The invention has been made under the circumstances described above.
One object of the invention is to provide novel iron sulfides which have excellent durability and are highly effective in treating heavy metals.
Another object of the invention is to provide processes for synthesizing these novel iron sulfides.
Still another object of the present invention is to provide iron sulfide mixture.
Further object of the invention are to provide a heavy-metal treating agent comprising either of the iron sulfides as an effective component.
Still another object of the invention is to provide a method by which heavy metals contained in an ash, soil, wastewater or the like are made harmless with the treating agent.
Intensive investigations have been made to overcome the above-described problems, i.e., the drawback of the synthetic iron sulfides heretofore in use that those have poor durability although highly active in heavy metal treatment. As a result, it has found that the durability of an iron sulfide can be greatly improved by incorporating an alkaline earth metal into the iron sulfide at least in a given amount to convert the iron sulfide into an iron sulfide which contains an essential component having a novel composition represented by FeMxNySz (wherein M represents an alkaline earth metal, N represents an alkali metal, and x, y, and z, indicating the molar proportions of the respective elements, represent numbers satisfying 0.01 less than xxe2x89xa60.5, yxe2x89xa60.2, and 0.7xe2x89xa6zxe2x89xa61.4). It has been further found that the novel iron sulfide is obtained by mixing an aqueous solution of a salt of bivalent iron, an aqueous solution containing sulfur ions and an alkaline earth metal ingredient and adjusting the pH of the resultant slurry to 7.0 or higher. Furthermore, it has been found that a heavy metal treating agent comprising this novel iron sulfide as an effective component is far more effective in treating various heavy metals than the synthetic iron sulfides heretofore in use and pyrrhotite iron sulfide. The invention has been completed based on these findings.